1. Technical Field of the Invention
The present invention is directed to Distributed Antenna Systems and more particularly, to methods and systems for compensating for signal loss or attenuation in order to provide predictable signal strength at the uplink and downlink endpoints.
Distributed Antenna Systems (“DAS”) are used to provide or enhance coverage for wireless services such as Cellular Telephony, Wireless LAN and Medical Telemetry inside buildings and over campuses. The general architecture of a DAS is depicted in FIG. 1.
A single DAS can serve a single wireless service or a combination of wireless services operating over multiple bands. With respect to each wireless service served by the DAS, the Aggregation Configuration of the wireless service can be characterized as non-aggregated or aggregated. In a non-aggregated configuration, there is a 1:1 relationship between DAS antennae and Transmitter/Receiver units for that wireless service. In an aggregated configuration, each Transmitter/Receiver unit for a given wireless service is associated with multiple DAS antennae through a hierarchy of aggregation. For example, in FIG. 2, Services A, B, and C are aggregated and Services D1, D2, and D3 are non-aggregated. Typically, wireless LAN services are arranged in a non-aggregated configuration when using a DAS while cellular services are typically arranged in an aggregated configuration.
One desired characteristic of a multi-service DAS is that it can use a single antenna to radiate and receive the signals for all services and frequency bands supported by the DAS. Such an antenna would need to cover (i.e. have acceptable performance) in all frequency bands of interest and is commonly referred to as a Broadband Antenna. An example of a supported frequency range for a DAS antenna would be 400 MHz-6 GHz.
In referring to the signal flows in DAS systems, the term Downlink signal refers to the signal being transmitted by the source transmitter (e.g. cellular base station) through an antenna to the terminals and the term Uplink signal refers to the signals being transmitted by the terminals which are received by an antenna and flow to the source receiver. Many wireless services have both an uplink and a downlink, but some have only a downlink (e.g. a mobile video broadcast service) or only an uplink (e.g. certain types of medical telemetry).
2. Description of the Prior Art
Different DAS may use different types of cabling to connect the antennae units to the wiring closet: various kinds of coaxial cable, CAT-5/6, optical fiber, etc. Analog signals can become attenuated as they propagate along the cable—the magnitude of attenuation depends on the characteristics of the cable, and is generally proportional to the length of the cable and to the frequency of the signal.
Multi-service DAS usually use passive (i.e. un-powered) broadband antennae located, for example, in the ceilings, throughout a facility, connected with broadband coax cabling to active components residing in wiring closets. This is because passive antennae are cheaper and more reliable than powered antennae and the introduction of power amplifiers in the antenna can introduce interference between bands that requires the use of bulky and expensive filters to mitigate.